A variety of apparatus and methods exist for producing glass/polymer composite strands, from which finished composite products are intended to be made. Methods such as solution processing, slurry processing, and melt impregnation involve passing a finished glass fiber tow or yarn through a polymer-containing liquid. The polymer clings to the tow and results in a composite preimpregnated ("prepreg") tow. Yet another method, called dry powder impregnation, involves applying a thermoplastic powder to a glass fiber tow and then applying heat to sinter the powder particles to the glass fibers.
All of the above-mentioned methods have various disadvantages. A common disadvantage exhibited by the above methods is the need for one or more off-line processing steps of spreading the glass tow and applying the polymer to the glass tow to produce a suitably commingled composite strand. Such off-line steps increase the complexity and cost of producing the composite strand, and thus the cost of a finished composite product made therefrom.
Another method for producing a glass/polymer composite strand includes commingling glass and polymer fibers using an in-line process. Such a process is disclosed in U.S. Pat. Nos. 5,011,523 and 5,425,796. Methods such as that disclosed in the 5,011,523 patent have several disadvantages. Of particular interest is the problem of the location of the polymer fibers within the finished strand. Specifically, the disclosed composite strand does not have polymer and glass fibers evenly distributed throughout the strand cross section. Rather, the polymer fibers, which will eventually form the thermoplastic matrix of a finished composite product, merely encompasses a single tow or multiple split tows of the glass fibers. Thus, the composite strand is more likely to contain relatively large grouping of glass fibers which are substantially devoid of the polymer fibers (i.e., the thermoplastic matrix material).
Methods such as that disclosed in the U.S. Pat. No. 5,425,796 patent also have several disadvantages. It has been found that because only the glass fibers are sized before the glass and polymer fibers are combined, the polymer fibers are less likely to be bonded to one another or to adjacent glass fibers. In addition, the amount of size that is used is typically very small. That is, the dry weight content of size in the resulting composite strand is believed to be less than about 1.0%. Therefore, a composite strand formed according to such a process is more likely to include glass and polymer fibers that are weakly bound together, if at all. With such weak or, in some areas, nonexistent bonding of the fibers, such a composite strand is less likely to be sufficiently coherent, and remain so, during subsequent processing into a finished composite product.
Accordingly, there is a need for an improved process for forming a glass/polymer composite strand having glass reinforcing fibers and matrix polymer fibers that are more evenly distributed and more fully bonded together throughout the composite strand cross section, such that it is more likely that the resulting composite strand will be and will remain sufficiently resilient and coherent to be subsequently processable into a finished composite product having optimized properties. There is also a need for a more efficient and economical process for forming a unidirectional fabric with a glass/polymer composite strand and, subsequently, for making a finished composite product using the resulting glass/polymer composite unidirectional fabric.